I have a 1-1/2 year old BorderCollie/AustralianShephard pup, Morgan, who has been very socialized since he was born. He loves company, playing with children and is accepting of almost anyone's attention until recently. Now it seems that for no reason that I can determine, he will become 'aggressive' towards certain people who come to the house, or that we meet on walks. They seem to always be male. He has never bitten anyone, although he has snapped at one guy. He usually just bares his teeth and growls and sometimes barks if they try to approach him...and he won't go away from them or leave them alone. I don't have a problem with him not liking someone and have never punished him for his reactions, but I want to teach him to accept them on my say-so and at least leave them alone and just be on guard if he doesn't want to befriend them. I've tried having them feed him snacks, distracting him to another room and all the techniques I've read about, but he keeps returning to growl at them! I'm not sure what sets these people off from the room full of other folks that he has no problems with, but I'm open to suggestions or at least an understanding of what's going on and why. I've never written on a forum before, but I'm hoping there's someone out there who can relate. Thanks in advance to anyone who replies!